From JP, A(U) (Japanese Utility Model Publication under Art. 13 bis of the Japanese Utility Model Law) No. 60-65453 and JP, A(U) No.61-135047, there is known a key-shift transmission in which shift keys slidably disposed within elongated axial grooves in the outter surface of a transmission shaft are connected by pins rotatably to a shift collar, slidably mounted on a transmission shaft for shifting the shift keys axially of the shaft, and are biased by compression coil springs accomodated within the shift collar to rotate towards a direction such that gear-engaging lugs of the keys are projected radially outwardly of the transmission shaft.
As compared to a structure known from, for example, U.S. Pat. Nos. 2,395,433 and 3,028,763 that a leaf spring affixed at one end thereof to a shift key is provided for biasing the key within a key-receiving axial groove in the outer surface of a transmission shaft, each of the shift keys employed in the key-shift transmission referred to above may be enlarged in thickness in the radial direction of transmission shaft so as to enlarge strength of the key. This is because compression coil springs disposed outside the axial grooves in a transmission shaft does not require a reduction of thickness of the shift key.
Of the Japanese Utility Model Publications referred to above, JP, A(U) No. 60-65453 discloses a transmission in which key-biasing compession coil springs are accomodated within a shift collar in a posture such that the coil springs extend radially of the shift collar. Contrarily to this, compression coil springs employed in the transmission shown in JP, A(U) No. 61-135047 are disposed within a shift collar so as to extend axially of the collar so that the coil springs do not require so much an enlargement of the outer diameter of a shift collar for disposing the springs within the collar.
However, a large biasing force is required for shift keys which operate to couple speed-change gears to a transmission shaft in a speed-change mechanism for vehicle. It is considered that the structure shown in JP, A(U) No. 61-135047 will also require a considerable enlargement of the outer diameter of a shift collar for locating pivot pins for the shift keys at radial positions where the interval between each pivot pin and a spring-acting point to each key is large enough for securing a required key-biasing force from the compression coil springs employed. Such enlargement of the outer diameter of a shift collar which rotates in unison with a transmission shaft will provide limitations to the arrangement of transmission members so that compactness of the transmission is impeded.